In reciprocating internal-combustion engines with spark ignition and direct fuel injection, the fuel is injected directly into the working space of the engine by means of an injector. With respect to the time of this injection, there are two basic operating modes.
In so-called homogeneous operation, the fuel is injected early into the combustion chamber, in general during the inflow of the combustion air, i.e. when the intake valve is open. Good homogenisation of the fuel/air mixture is thereby achieved. This operating mode is favourable when the engine is operating at high load.
In so-called stratified-charge operation, injection does not take place until after the gas intake valve has closed, when the piston passes into the region of its top dead centre position during its upwards movement. The intention here is that the fuel is only mixed with some of the fresh air contained in the cylinder and also only in a locally limited manner before it is ignited by the ignition device. This operating mode is preferably employed when the engine is operating at partial load or no load. The advantage is that the engine can be operated without throttling the intake air and without the fuel-to-air ratio in the vicinity of the ignition device being too lean for reliable ignition.
For these operating modes, different methods of introducing the fuel into the cylinder chamber and forming the mixture have become known and can be divided into two categories:
In the so-called jet-guided methods, the injection jet is aimed directly at the ignition device. The injected fuel cloud mixes with the combustion air and is ignited by the ignition device. Accordingly, reliable stratified-charge operation is only ensured if the ignition device is positioned very close to the injector. This has the disadvantage that only an extremely small, operating-point-specific ignition window is available and, accordingly, adjustment of the jet diffusion for large performance characteristic ranges is critical. In addition, the injectors used have to be manufactured with great precision. Even small tolerances or changes in the injector during long-term operation lead to disadvantageous boundary conditions for ignition.
The ignition conditions in stratified-charge operation can therefore only be ensured by an exact geometric relationship between the ignition device and the injection jet. Consequently, the known methods in this category are carried out without a defined or strong charge movement. In homogeneous operation, however, this movement for improving the homogenisation of the fuel/air mixture is absent, resulting in output losses and an increase in fuel consumption with correspondingly increased pollutant emissions.
This method has the further disadvantage of increased wear and shortened life of the ignition devices caused by direct injection onto the ignition device.
The so-called wall-guided methods are based on the fact that, in stratified-charge operation, the fuel injection jet is deflected onto the injection device by the part of the combustion chamber wall formed by the piston crown. This is assisted by a strong charge movement. This method avoids direct injection onto the ignition device. Tolerances and the operating state of the fuel injectors are less critical than in the above-described jet-guided methods.
A disadvantage of these methods is that, when the fuel is injected directly into the cylinder chamber, it travels as far as a combustion chamber wall, in particular the piston crown, with the result that in certain operating states there is incomplete combustion, leading to increased emission of unburned hydrocarbons and increased emission of soot. This method has hitherto been carried out with a fuel injector on the intake side and, from the point of view of orientation and direction of rotation, is based on the formation of a specific cylindrical flow of the cylinder charge in the injection direction, which guides the mixed jet to the ignition device via the piston crown. This pattern of the charge movement can be achieved by steeply upright intake ports (EP 0 558 072 B1 and DE 197 08 288 A1), resulting in a correspondingly greater overall height of the engine. According to another proposed solution, the desired movement pattern of the cylinder charge is achieved by a particular formation of the intake port or, for example, the geometry in the seat region of the intake valve (EP 0 463 613 B1), although this has disadvantageous effects on the flow quality of the intake system and thus on the full-load operation of the engine. In both cases, the injection jet is aimed at a recess in the piston crown so that, during stratified-charge operation, fuel which is still liquid strikes the piston crown. The mixture forming there is then guided towards the ignition device in contact with the wall of the piston crown.
With this method, the strong charge movement necessary in stratified-charge operation has a disadvantageous effect during homogeneous operation owing to the resulting harsh combustion noises and increased wall heat losses.
WO01/49996A discloses a reciprocating internal-combustion engine having two gas in-take ports with gas intake valves and at least one gas exhaust port with a gas exhaust valve and at least one ignition device per cylinder, and having a combustion chamber formed by the cylinder cover and the crown of the piston, which is guided in the cylinder in a reciprocating manner, wherein the piston crown on the one hand and the cylinder cover on the other hand are substantially roof-shaped in vertical section, and wherein in each case one of the roof surfaces is associated with the gas intake valves and the other roof surface is associated with the gas exhaust valve, and the orientation of the roof surfaces of the cylinder cover corresponds to the orientation of the roof surfaces of the piston crown. A trough-shaped recess is provided in the piston crown and extends over the region of the roof ridge on the piston crown and thus over both roof surfaces. The fuel injection nozzle opens into the cylinder near the inlet region of the gas intake port next to the gas intake valve. The ignition device is arranged in the cylinder cover close to the vertical cylinder axis.
Owing to the roof-shaped formation of the cylinder cover and also the piston crown, a combustion chamber is formed which is only slightly creviced so that, in conjunction with the trough-shaped recess in the piston crown, movement of the cylinder charge towards the fuel injection nozzle is produced. As the gas intake ports open into the combustion chamber at an angle, a tumble flow is formed during the intake stroke and initially flows into the combustion chamber along the cylinder cover and is then guided back towards the fuel injection nozzle by the trough-shaped recess. The flow is also maintained during the compression stroke.
In particular during stratified-charge operation, air is guided towards the fuel injection nozzle via the piston crown, and the fuel is then injected into the air. Improved mixture preparation is achieved in the immediate vicinity of the gas intake valve if, during stratified-charge operation, fuel injection takes place after closure of the intake valve at a time at which the piston is close to its top dead centre position during its upwards movement. In the region of the injection valve, the air flow changes direction and is guided towards the ignition device. In spite of the reduced combustion chamber volume, a long free jet path with optimum mixture formation is then available for the fuel in the direction of the air flow in the cylinder towards the ignition device. This produces only a slight coating of fuel on the cylinder walls. The particular formation of the combustion chamber in combination with the position of the fuel injection nozzle permits very flat guiding of the jet, enabling the fanned-out jet to enter the combustion chamber in the region of the trough-shaped recess in the piston crown in a satisfactory manner and almost without wetting the piston crown, also during stratified-charge operation, so that an optimum fuel/air mixture reaches the ignition device.